1. Field of the Invention
This invention relates generally to a replication technique for use in reproducing surface contours in devices, such as microdevices. Specifically, the present invention is directed to a method of making a master wafer whereby the master wafer is replicated and used to produce hi-fidelity copies.
2. Description of Related Art
Conventional gray scale etching processes enable the creation of various shapes and curves, both symmetric and asymmetric, thereby lending itself well for the use in fabrication of micro-optical systems. Traditional binary optics, such as RIE (Reactive Ion Etching) and DRIE (Deep Reactive Ion Etching) utilizes step etches which are used to approximate a smooth curve. However, gray scale etching processes can actually create smooth curves such as smooth ramps, lens, torroids, turbo machines, gears, and other complex shapes.
The utilization of gray scale optics in the formation of products such as microlenses is typified in U.S. Pat. No. 5,310,623 to Gal., for example. In particular, the Gal patent pertains to a microlens replica formed in the photoresist material with a gray scale mask of a selected wavelength (usually in the UV), and the material replica is subsequently used to reproduce the replica directly in the substrate by differential ion milling.
Differential ion milling entails shooting a stream of argon atoms at the substrate, which is often considered to be analogous to sandblasting. As a result, differential ion milling is amenable to the formation of curved micro-structures. However, differential ion milling has the disadvantage of not being able to readily perform the deep etching necessary to form the structures necessary for micro-lenses or turbine rotors.
Standard gray scale techniques, such as that of Gal, allow the etching of shallow (<100 μm) structures, but have the disadvantage of needing thick layers of photoresist, wherein approximately 20 μm layer of photoresist is required to produce a 100 μm structure, for example. The gray scale lithography process may also be used to produce improved molds for producing micro-optical elements. The gray scale lithography process offers smoother curves and more flexibility in the heights and depths that may be formed in the substrate than those offered by conventional techniques.
Consequently, the optical elements produced by this process also exhibit improved characteristics both in surface finish and other characteristics of the finished optical elements.
The gray scale lithography process uses a gray scale mask to pattern a photoresist on a substrate, which is subsequently etched to form curved shapes. Patterning the photoresist to form a photomask layer can be performed using a single gray scale mask. Alternatively, patterning the photoresist to produce a variable thickness photomask layer can be accomplished by exposure with 2 gray scale masks.
By using an appropriate pattern, an exposure in a photoresist material can be created which will cause the height of the processed photoresist material to replicate the height of the desired workpiece. The exposed photoresist can then be processed using known methods to produce an impression of the desired pattern in the developed photoresist. Alternatively, the patterned photoresist itself can be the final product.
The image impression in the photoresist is produced by exposing the photoresist material to light of a selected wavelength through the gray scale mask, transmitted through openings in the exposure mask for a selected time period. The light is usually ultraviolet light. The exposed photoresist material is subsequently processed to procure the desired object on a substrate material using an etching method such as RIE (Reactive Ion Etching) or DRIE (Deep Reactive Ion Etching).
Gray scale mask technologies, including the half tone process, the modulated exposure masking technique and Canyon Material's High Energy Beam Sensitive (HEBS) glass, can be used to imprint the photoresist. These techniques partially expose a photosensitive material to achieve a desired structure. Photosensitive materials include, but are not limited to, photoresist and PMMA (polymethyl methacrylate) materials. When using HEBS, the glass itself is photosensitive.
High Energy Beam Sensitive (HEBS) glass is a one step fabrication of a gray level mask. The exposure of this gray level mask is done using an e-beam writing tool. The e-beam writing tool software is used to support mask making and direct write-on resist approaches for the fabrication of diffractive optical elements (DOEs). The so-generated gray level mask is usable in an optical exposure tool (e.g., a G-line stepper, or a contact printer) to mass fabricate resist profiles.
Using the HEBS-glass gray level mask fabrication and a following optical exposure, alignment errors are possibly avoided, since the mask is written in a single step using different electron beam dosages to generate gray levels. Instead of fabricating of a set of five binary masks with all the involved resist processing and wet etching, only a single writing step without any resist processing is used. This single mask then contains all the necessary information previously contained in a set of five binary chrome masks.
After the HEBS gray level mask is fabricated a series of single exposures in a step-and-repeat system can generate hundreds of DOEs on the same wafer. This wafer can then be processed to transfer the DOE structure of a large number of different elements into the substrate. Since the complete DOE structure is transferred into the substrate there is no need for a resist stripping step after the etching process. After dicing the wafer, many of monolithic multilevel DOEs have been generated.
However, there are at least two main sources of errors that plague the surface profile of structures, in photosensitive materials, resulting from a gray scale or analog lithography process.
The first source of error arises from general roughness in the surface of the photosensitive material. This error may be caused by the slight variations in the dose of the writing tool, usually an electron beam (e-beam) or laser. In the case of the half tone process, the chosen pixel shape scheme may cause this error. The period of oscillation for the general roughness error is typically on the order of 10 microns.
The second source error is the stitching error, i.e., it is geometric and is induced by slight variations in the positioning and size of the writing tool. Stitching error is due to slight inaccuracies of the stage and field of the writing tool. The stage of the writing tool refers to the horizontal sweep, wherein slight variation in the positioning of the horizontal line results in stitching error. The field refers to the width of the writing line, wherein variation in the width of the writing line also results in stitching error. The stitching error is of low frequency period and manifests itself in the slight vertical lines in the surface
In other situations, typically, several wafers are used in order to develop a manufacturing process usable to obtain desired surface characteristics on a series of wafers, including a desired surface shape, surface roughness, wafer uniformity, focal length tolerance, etc. However, once a first wafer is processed, only the general parameters for the process are established. As a drift occurs in the process, i.e., as a result of etching, the parameters must be re-adjusted to compensate for the drift. This requires that several more wafers be processed to determine the new parameters and produce objects with the desired features.
Unfortunately, the patterning process also drifts and the associated parameters must be adjusted to compensate for this drift. The combination of etching and patterning process variants unfortunately translates into extremely low yield. For this reason, for customers requiring high-volume, the issues of cost, consistency, and quality become significant.
Hence, there is a need to provide a first wafer that meets all requirements and designate such a wafer as a “master” whereby the master wafer will be replicated and used to produce copies.